Optical interferometric sensors have enjoyed a wide variety of applications due to their high measurand sensitivity. The measurand can be any physical parameter which perturbs the effective refractive index, n.sub.eff, or which perturbs the length of the sensing arm or optrode of the interferometer. Common measurands include temperature, pressure, acoustic energy, acceleration, force, electric and magnetic fields, rotation rate, fluid level, fluid flow rate, and the concentration of specific chemicals on or near a surface.
High measurand sensitivities are achieved by these optical interferometric sensors by use of long interferometer path lengths which can be realized with available low-loss single-mode optical fiber. However, a persistent problem with the application of fiber optic interferometric sensors has been that a sensor built to sense one particular measurand also responds to some degree to variations in other environmental variables (for example, pressure or temperature), and also responds to small variations in the wavelength of the light source used to power the interferometer.
In known interferometric sensors, for example, the Mach-Zehnder sensor (FIG. 1), and Recirculating Delay Line sensor (FIG. 2), increasing measurand sensitivity by using a longer optical path length for the sensing arm results in an undesirable increase in sensitivity to variations in the wavelength of the light source powering the sensor, and an undesirable increase in sensitivity to other environmental variations such as ambient pressure and temperature which affect n.sub.eff.
In the Mach-Zehnder sensor of FIG. 1, which includes a sensing arm, L1, a reference arm, L2, and two evanescent couplers, C1 and C2, the sensitivity to these "commonmode" variations in source wavelength or effective index in both arms of the Mach-Zehnder sensor can be reduced by making the sensing arm length, L1, nearly equal to the reference arm length L2, but at the price of reduced measurand sensitivity, Ss. In fact, when L1 equals L2, common-mode fluctuations are completely compensated for in the Mach-Zehnder sensor, but measurand sensitivity Ss is also reduced to zero.
Since the Recirculating Delay Line sensor (FIG. 2) is actually composed of a single optical path, Lf, through a resonant cavity including evanescent coupler, C3, no common-mode compensation is possible. In the Recirculating Delay Line device, sensitivity to the measurand can be increased relative to that of the known Mach-Zehnder interferometer by operating the sensor near cavity resonance, but sensitivity to environmental and wavelength fluctuations is also increased.
The tradeoffs between increased measurand sensitivity and increased undesirable common-mode variation in the Mach-Zehnder and Recirculating Delay Line sensors, which are transmission-type sensors, also exist in reflection-type sensors, such as the Michelson-Morley sensor (FIG. 3), and the Fabry-Perot sensor (FIG. 4). The Michelson-Morley sensor of FIG. 3 includes sensing arm, L4, reference arm, L3, evanescent coupler, C4, and mirrors M1 and M2. Mirror M1 is fixed and mirror M2 can be moveable responsive to changes in a measurand, or both mirrors M1 and M2 can be fixed and an optrode can be included in sensing arm, L4. The Fabry-Perot sensor of FIG. 4 includes a resonant optical path including arms L5 and L6, evanescent coupler, C5, and mirrors M3 and M4. Mirror M3 is fixed and mirror M4 can be moveable responsive to changes in a measurand, or both mirrors M3 and M4 can be fixed and an optrode can be included in arm L6. In the Michelson-Morley sensor, similar to the Mach-Zehnder sensor, undesirable common-mode variation can be eliminated, but not without sacrificing measurand sensitivity. In the Fabry-Perot sensor, as in the Recirculating Delay Line sensor, common-mode compensation is not possible, and increasing measurand sensitivity by operating the sensor near cavity resonance necessarily increases undesirable common-mode variation.
In the Figures, the optical input is designated by, i, and the optical output is designated by, o. Also, optical paths are depicted with bold lines, and electrical paths are depicted with fine lines.